It has been generally accepted that the electroerosion technique, when applied to the machining of a 3D cavity in a workpiece, commonly requires a tool electrode three-dimensionally shaped to conform to the desired cavity in the workpiece. Furthermore, a multiplicity of such electrodes of identical or similar configuration and size must be prepared in order to compensate for the wear which the electrodes or tools suffer during the erosion process or in the interest of minimizing the machining time to achieve desired machined precision and surface finish. Preparation of such precision-formed and multiple electrodes is obviously time-consuming and laborious. Furthermore, the conventional sinking-type operation makes it difficult to keep the machining gap free from the debris of machined chips and other products which tend to cause machining instability.